1. Field of the Invention
The present invention relates to an image processor and an image processing method for determining formation volumes of a high-density dot and of a low-density dot for a printing medium, a printing apparatus and a printing method that use the same, a program, and a storage medium that stores a computer-readable program code.
2. Description of the Related Art
As the conventional ink-jet printing apparatus, there is, for example, an apparatus that uses three color inks of cyan (C), magenta (M), and yellow (Y), or four color inks of cyan, magenta, yellow, and black (K) and forms a dot by applying these inks with a printing medium. In such a printing apparatus, there is a case where the dots become noticeable in a highlight portion (low-density area) of an image, resulting in large granularity. As a method for reducing such granularity of the dots, there is a method where a light ink whose ink density is as small as ⅓ to ⅙ or so of those of the normal inks is also used for the inks of low intensity, such as cyan and magenta. As the printing method using the light inks, there are the following three schemes.
(1) A highlight portion is applied with the light ink, and a shadow portion (high-density area) is applied with the light ink by overlap.
(2) The highlight portion is applied with the light ink, and the shadow portion is applied with a dark ink.
(3) The highlight portion is applied with the light ink, the medium-density portion is applied with a medium-density ink, and the shadow portion is applied with the dark ink.
In the printing style of the above-mentioned case (1), consumption of the light inks becomes larger as compared to the normal printing and hence a running cost becomes high, because the density in the shadow portion is intended to be deepened by ejecting the light ink in an overlapping manner. In addition, the applying amount of the inks is increased to ensure the density of image, and increase of the applying amount of the inks causes the inks to overflow on a printing medium, especially when printing is done for secondary colors, tertiary colors, etc., resulting in deterioration in image quality. Moreover, because of the existence of the maximum possible applying amount of the ink, available printing mediums are limited.
In the printing style of the above-mentioned case (2), by using inks properly for the highlight portion and the shadow portion, coexistence of reduction in the granularity in the highlight portion, and enhancement of the density in the shadow portion as well as reduction of ink consumption are intended to be established. However, in the case where the concentration of the light ink is reduced as thin as possible, although the granularity in the highlight portion becomes small, the dots become noticeable in a medium-density range where the dark ink starts to mingle in an area formed by the dots of the light ink because of existence of a large difference between the densities of the light ink and of the dark ink on a printing medium. On the other hand, in the case where the density of the light ink is increased with the intention of reducing the granularity of the dots in the medium-density range, the dot of the light ink becomes noticeable and the granularity increases in the highlight portion because the dots come into prominence there.
In the printing style of the above-mentioned case (3), by using the medium-density ink, the granularity of the dots becomes small in the medium-density range. However, it is necessary to prepare both three printing heads each for ejecting one of the light ink, the medium-density ink, and the dark ink, and three types of the inks for a single hue and it becomes a major factor of increase in cost. In addition, when performing image processing, it is necessary to provide three tables of color for a single hue, which leads to complicated image processing.
Moreover, as a printing style that does not use the light inks, for example, there is a method such that an ink droplet for forming the dot is reduced to a small droplet of approximately 0.5 pl (pico liter) to form a dot smaller than that of the normal case, achieving reduced granularity in the highlight portion, with the intention of achieving the same image quality as the above-mentioned cases (1), (2), and (3) that use the light inks. However, this printing method may result in a reduced printing speed because of increased printing resolution, increased difficulty in stably ejecting the ink droplet at a targeted impact position, or the increase in cost because of difficulty in manufacturing the printing head and hence depression of its yield.